I. Field of the Invention
The present invention relates to a process for preparing an aqueous copolymer solution from which is removed the residue of a catalyst. More particularly, it relates to a process for preparing an aqueous solution of a copolymer from which is removed tetramethylsuccinonitrile (hereinafter referred to as "TMSN"), that is, a residue formed as a by-product in the decomposition of azobisisobutyronitrile (hereinafter referred to as "AIBN") in preparing the copolymer aqueous solution, without separating the copolymer from a polymerization liquid in which a diolefin-maleic anhydride copolymer is synthesized by means of solution polymerization using AIBN as a polymerization initiator.
II. Brief Description of the Prior Art
Maleic anhydride series copolymers have gained a special position as useful water-soluble polymers because of their characteristic properties with respect to easy modification, ready solubility in water and easily varied physical properties with variation in the copolymerizable monomers used. Among such copolymers, diolefin-maleic anhydride copolymers are useful copolymers possessing various possibilities because the double bonds remain in the molecule, in addition to residues of the maleic anhydride used, and the double bonds can be utilized for modification or cross-linkage. Maleic anhydride series copolymers are generally synthesized by means of slurry polymerization or solution polymerization in a water-insoluble solvent using a radical polymerization initiator such as a peroxide or an azo compound. However, it is known that, where diolefin-maleic anhydride copolymers are synthesized by the slurry polymerization method in which the polymers produced are caused to precipitate during polymerization, the polymers are gelled. Accordingly, the soluble polymer can be produced only by the solution polymerization method.
Where diolefin-maleic anhydride copolymers are employed as water-soluble polymers, they are usually employed in the form of an aqueous solution of the copolymer which is obtained by reacting the copolymer with water for the conversion of maleic anhydride residues into maleic acid residues to render the copolymer soluble in water, or by reacting the copolymer with a base such as sodium hydroxide or the like for the conversion of the maleic anhydride residues into the maleic acid salt residues to render the copolymer soluble in water. Where such an aqueous copolymer solution is prepared from the diolefin-maleic anhydride copolymer synthesized by means of solution polymerization, the desired aqueous solution may be obtained on a small scale by pouring the polymerization liquid into a barren solvent of the copolymer to separate the copolymer for purification and then reacting the resultant copolymer with water or with sodium hydroxide or the like in water. These procedures, however, present disadvantages in that a large amount of the solvents is required, and the operation is laborious so that they are difficult to practice industrially. In order to obtain the aqueous copolymer solution on an industrial scale, it is desired to provide a process for preparing the aqueous solution directly from the polymerization liquid without separating the copolymer produced from the polymerization liquid in which the copolymerization was carried out. However, the drawback encountered by such a procedure is that it is difficult to provide an aqueous copolymer solution from which unreacted monomers, reaction by-products and residues of a catalyst used are removed from the polymerization liquid in which copolymerization is effected.
Although AIBN is cheap in cost, stable and high in safety, it is hard to remove TMSN, that is a by-product resulting from the decomposition of AIBN when the solution polymerization is effected with the aid of AIBN as a radical polymerization initiator. As TMSN is high in toxicity, the use of the aqueous copolymer solution from which TMSN is not sufficiently removed is restricted from the safety point of view. Accordingly, the removal of TMSN always presents a big problem where an aqueous solution of a diolefin-maleic anhydride copolymer is prepared directly from the polymerization liquid in which the copolymerization is carried out.